You’re confusing lots of different things here.
The sponge tool works the same way the color and luminosity blend modes work.
And the exact weighting factors are dependent on the RGB color space in use — but the blend modes and sponge tool use an approximation instead of the exact weights.
Hi Chris,
Thanks for responding on this, but the subject was discussed in much more detail in a related thread initiated by david Kroll on Jan 29, topic "Sponge Tool Problem-Photoshop 6.01 on WindowsXP". If you have time, it would be nice to get your critique after reading that thread.
Thanks, George
dave milbut "Sponge Tool problem — Photoshop 6.0.1 on Windows XP" 1/29/04 9:10am </cgi-bin/webx?14/0>
just watching this one. thanks for the help in the other thread george.
This contradicts the definition of luminosity which, as a measure of perceived brightness
I always thought luminosity was _measured_ brightness (as opposed to _perceived_ brightness)
(To demonstrate what I mean, draw a 50% grey line through a black to white gradient. This gives a well known optical illusion where the perceived luminosity of the line will change, but its measured value will stay the same)
Rene
"…I always thought luminosity was _measured_ brightness (as opposed to _perceived_ brightness)…"
I don’t know. My thought has been that "Luminance" was measured intensity and "Luminosity" perceived.
Your experiment is a striking demonstration that the eye-brain is doing its job, which is to INTERPRET what’s seen—not measure it.
George
I don’t know. My thought has been that "Luminance" was measured intensity and "Luminosity" perceived.
Could be, I’ll have to look it up at a more reasonable time…
Rene
Whatever source you go to for definitions, keep in mind that applications do not necessarily use universally accepted definitions or universality may not have been established for a particular parameter.
When using Photoshop, we’ve got to go with Photoshop’s definitions whether or not they are consistent with general usage, a particular consortium’s agreement, international standards, etc. Where Photoshop does not explicitly define a parameter, we have to deduce it. Photoshop’s luminosity is a case in point. Going to Image > Histogram and reading mean luminosity for a uniform patch (selected, so the readings apply only to the patch) lets us relate PS luminosity to the component color channel values and reveals how PS weights those components. There can be no argument with any result so obtained.
Your mid-gray line drawn across a black-to-white gradient demonstrates that perceived luminosity for a constant-intensity, constant-color specimen will vary with the background against which it is viewed, which emphasizes the necessity for keeping that background constant when establishing weighting factors for RGB components. Clearly, a constant background would have to have been used in calibrating the relative RGB brightness perceptions.
George
Dave, Chris, Rene
I am sure no-one else would possibly check this thread buried as it is below tons of later topics.
In mulling over this issue, I have changed my thinking to be consistent with the facts. Here’s how I squirm out of the dilemma:
One is being much too literal to expect perceived brightness to remain constant by simply constraining changes in the 3 channels to keep the invented parameter "luminosity" (.3R + .59G +.11B) constant. Such a constraint might be expected to mute the eye’s unbalanced sensitivity, but hardly to do so perfectly. It is likely impossible to find any set of constant weights that will do this job over the entire range of intensity variations. Using the chosen weights HELPS but cannot do the miraculous.
Therefore, in applying the constraint that luminosity be held constant, one should not expect perceived brightness to be unchanged. Just not changed so much over limited ranges. It’s better than not trying at all to weight the values to compensate for the eye’s favoritism. To be sure, one can keep the defined luminosity constant in an editing operation such as sponging—and this is indeed what PS does—but look only for compensation not total elimination of perceived brightness changes. My mistake is looking for perception to rigidly mimic the parameter’s constancy.
George
